Modular stacked dc architecture traction system and method of making same

ABSTRACT

A modular stacked DC architecture for traction system includes a propulsion system includes an electric drive, a direct current (DC) link electrically coupled to the electric drive, and a first DC-DC converter coupled to the DC link. A first energy storage device (ESD) is electrically coupled to the first DC-DC converter, and a second DC-DC converter is coupled to the DC link and to the first DC-DC converter. The system also includes a second energy storage device electrically coupled to the second DC-DC converter and a controller coupled to the first and second DC-DC converters and configured to control a transfer of energy between the first ESD and the DC link via the first and second DC-DC converters.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a continuation of and claims priority to U.S.patent application Ser. No. 13/307,191 filed Nov. 30, 2011, thedisclosure of which is incorporated herein in its entirety.

BACKGROUND OF THE INVENTION

Embodiments of the invention relate generally to hybrid and electricvehicles and, more particularly, to a modular stacked direct current(DC) architecture traction system for hybrid and electric vehicles.

A hybrid electric vehicle (HEV) may combine an internal combustionengine and an electric motor powered by an energy storage device, suchas a traction battery, to propel the vehicle. Typically, the electricmotor of an HEV is coupled between the internal combustion engine andthe transmission to take advantage of the torque increase through thetransmission. Such a combination may increase overall fuel efficiency byenabling the combustion engine and the electric motor to each operate inrespective ranges of increased efficiency. Electric motors, for example,may be efficient at accelerating from a standing start, while combustionengines may be efficient during sustained periods of constant engineoperation, such as in highway driving. Having an electric motor to boostinitial acceleration allows combustion engines in HEVs to be smaller andmore fuel efficient.

A purely electric vehicle (EV) typically uses stored electrical energyto power an electric motor, which propels the vehicle. EVs may use oneor more sources of stored electrical energy and are configured to useenergy from an external source to re-charge the fraction battery orother storage devices. For example, a first source of stored energy(sometimes referred to as an “energy” source) may be used to providelonger-lasting energy while a second source of stored energy (sometimesreferred to as a “power” source) may be used to provide higher-powerfor, for example, acceleration from standstill or boost duringoperation. First and second sources may include chemical-based batteriesor may include ultracapacitors, as examples. Typically, the source(s) ofelectrical energy (energy and/or power batteries) in EVs are charged viaa plug-in charger or other external energy source. With typicallycomplete reliance on plug-in power, an EV may have increased energystorage capacity as compared to an HEV.

A plug-in hybrid vehicle (PHEVs) may include both an internal combustionengine and an electric motor powered by an energy storage device, suchas a traction battery. Typically a PHEV is configured to use energy froman external source to re-charge the traction battery or other storagedevices. Thus, with increased reliance on plug-in power, a PHEV may haveincreased energy storage capacity as compared to an HEV.

There are generally two types of PHEV: parallel and series. In aparallel PHEV arrangement, the electric motor is coupled between theinternal combustion engine and the transmission, enabling the combustionengine and the electric motor to each operate in respective ranges ofincreased efficiency, similar to an HEV. In a series PHEV arrangement,the electric motor is coupled between an energy storage device and thevehicle drive axle, while the internal combustion engine is coupleddirectly to the energy storage device and not to the vehicle drive axle.The series PHEV may also be referred to as an extended range electricvehicle (EREV), in reference to a purely electric drive system havingenergy augmentation to the energy storage system via the internalcombustion engine and via, for instance, a liquid fuel storage system.

In general, EVs, HEVs, and PHEVs typically include regenerative brakingto charge the charge storage devices during braking operations. Also,such vehicles may include on-road and off-road vehicles, golf cars,neighborhood electric vehicles, forklifts, and utility trucks asexamples. These vehicles may use either off-board stationary batterychargers or on-board battery chargers to transfer electrical energy froma utility grid or renewable energy source to the vehicle's on-boardtraction battery.

While hybrid and electric vehicles offer many advantages, managing thestored energy efficiently and maintaining a good capacity of the energystorage (ES) elements over a defined period are important considerationswhen sizing these ES elements. A high voltage ES element having severalcells in series often experiences less-than-optimal cell voltagebalancing. Degradation of a single cell in a string of series cellsaffects the capacity of the entire string. Due to this reason, a lowvoltage ES with several parallel strings and low number of seriesstrings is often preferred.

In contrast to the low voltage preferred on the ES, the EV motortypically meets the torque and efficiency requirements when operated athigh voltages. Often, DC-DC boost converters are used to couple lowvoltage ES to the high voltage DC link from which the drive of the motoris operated. These DC-DC converters typically employ expensive highvoltage switches that are rated for the DC link voltage.

It would therefore be desirable to provide an apparatus for coupling thelow voltage ES to the high voltage DC link using less-expensive, lowvoltage switches.

BRIEF DESCRIPTION OF THE INVENTION

In accordance with one aspect of the invention, a propulsion systemincludes an electric drive, a direct current (DC) link electricallycoupled to the electric drive, and a first DC-DC converter coupled tothe DC link. A first energy storage device (ESD) is electrically coupledto the first DC-DC converter, and a second DC-DC converter is coupled tothe DC link and to the first DC-DC converter. The system also includes asecond energy storage device electrically coupled to the second DC-DCconverter and a controller coupled to the first and second DC-DCconverters and configured to control a transfer of energy between thefirst ESD and the DC link via the first and second DC-DC converters.

In accordance with another aspect of the invention, a method ofassembling a control system includes coupling a first energy storagedevice (ESD) to a first DC-DC converter, coupling the first DC-DCconverter to a DC link, and coupling a second ESD to a second DC-DCconverter. The method also includes coupling the second DC-DC converterto the first DC-DC converter and to the DC link, coupling the DC link toan electric drive, coupling a controller to the first and second DC-DCconverters, and configuring the controller to cause the first and secondDC-DC converters to transfer energy between the first ESD and the DClink.

In accordance with another aspect of the invention, an energy storagearrangement for an electrically powered system includes a first energystorage device coupled to a first DC-DC converter, a second energystorage device coupled to a second DC-DC converter, and a DC link. TheDC link includes a first bus coupled to the first DC-DC converter and asecond bus coupled to the second DC-DC converter. The arrangementfurther includes a controller coupled to the first and second DC-DCconverters and configured to cause a current to flow from the second busto the first bus through the first energy storage device and through thefirst and second DC-DC converters.

Various other features and advantages will be made apparent from thefollowing detailed description and the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings illustrate embodiments presently contemplated for carryingout the invention.

In the drawings:

FIG. 1 is a schematic diagram of a fraction system in accordance with anembodiment of the invention.

FIG. 2 is a schematic diagram of the traction system shown in FIG. 1 ina motoring mode in accordance with an embodiment of the invention.

FIG. 3 is a schematic diagram of the traction system shown in FIG. 1 ina regenerative braking mode in accordance with an embodiment of theinvention.

FIG. 4 is a schematic diagram of the traction system of FIG. 1 inaccordance with another embodiment of the invention.

FIG. 5 is a schematic diagram of the traction system shown in FIG. 4 ina motoring mode in accordance with an embodiment of the invention.

FIG. 6 is a schematic diagram of the traction system shown in FIG. 4 ina regenerative braking mode in accordance with an embodiment of theinvention.

FIG. 7 is a schematic diagram of the fraction system shown in FIG. 4 inan energy transfer mode in accordance with an embodiment of theinvention.

DETAILED DESCRIPTION

FIG. 1 illustrates a modular stacked DC architecture traction/propulsionsystem 2 in accordance with an embodiment of the invention. Tractionsystem 2 includes a first energy storage device (ES1) 4 and a secondenergy storage device (ES2) 6. In embodiments of the invention, ES1 4and ES2 6 are low voltage, high specific-energy storage devices, andeach may be, for example, an ultracapacitor or an energy battery. Inthis case, an ultracapacitor represents a capacitor comprising multiplecapacitor cells coupled to one another, where the capacitor cells mayeach have a capacitance that is greater than 500 Farads. The term energybattery used in the embodiments shown herein describes a highspecific-energy battery or high energy density battery demonstrated toachieve an energy density on the order of 100 W-hr/kg or greater (e.g.,a Li-ion, sodium-metal halide, sodium nickel chloride, sodium-sulfur,zinc-air, nickel metal halide, or lead acid battery, or the like).

ES1 4 and ES2 6 are coupled to respective bi-directional DC-to-DCconverters 8, 10 that, in one embodiment, are configured in an H-bridgeconfiguration. Converter 8 includes a plurality of power switches S1-S4coupled in an anti-parallel arrangement with a plurality of diodesD1-D4. Likewise, converter 10 includes a plurality of power switchesS5-S8 coupled in an anti-parallel arrangement with a plurality of diodesD5-D8. Power switches S1-S8 may be, for example, bipolar junctiontransistors (BJTs) as shown, metal-oxide-semiconductor field-effecttransistors (MOSFETs), insulated gate bipolar transistors (IGBTs),silicon-controlled rectifiers (SCRs), contactors, or other powerswitches known in the art. Converters 8, 10 are coupled to respectivefirst and second buses 12, 14 of a DC current link 16, and an inductor18 is additionally coupled to first bus 12.

As shown in FIG. 1, DC current link 16 is coupled to a load 20, which,according to an embodiment of the invention, is an electric driveincluding an inverter 22 and a motor or electromechanical device 24. Inthis embodiment, inverter 22 is a current source inverter configured toconvert a current on DC current link 16 to an energy suitable fordriving motor 24. Motor 24 is preferably an AC motor but is not limitedas such. While not shown, it is to be understood that each of aplurality of motors 24 may be coupled to a respective wheel or otherload or that each motor 24 may be coupled to a differential fordistributing rotational power to the wheels or other load.

A controller 26 is coupled to switches S1-S8 of converters 8, 10 and toload 20 to control the transfer of energy from either or both of ES1 4and ES2 6 to load 20 during a motoring mode and to control the transferof energy generated during regenerative braking event to either or bothof ES1 4 and ES2 6 during a deceleration event. Additionally, controller26 may be configured to control the transfer of energy from ES1 4 to ES26 or from ES2 6 to ES1 4.

FIG. 2 is a schematic diagram of the traction system shown in FIG. 1 ina motoring mode in accordance with an embodiment of the invention. Inthis embodiment, controller 26 controls the transfer of energy from oneor both of ES1 4 and ES2 6 to load 20 for operating motor 24 in themotoring mode. As illustrated in bold, controller 26 allows the transferof energy from both ES1 4 and ES2 6 to DC current link 16 by maintainingswitches S1, S4, S5, and S8 in an off state and by controlling the dutycycle of switches S2, S3, S6, and S7. In this manner, current flowingfrom second bus 14 of DC current link 16 flows along a path throughswitch S7, ES2 6, switch S6, switch S3, ES1 4, and switch S2 and tofirst bus 12 of DC current link 16. The path of current from first bus12 of DC current link 16 to second bus 14 of DC current link 16 flowsthrough switch S10 of converter 28, through load 20, and through diodeD11.

While the embodiment illustrated in FIG. 2 shows control of switches S2,S3, S6, and S7 of converters 8 and 10 to control the transfer of energyfrom both ES1 4 and ES2 6, by controlling the duty ratio of switches S2and S6, controller 26 can also regulate the power to be drawn only fromone of the energy storage elements, i.e. either from ES1 4 or from ES26. For example, by controlling duty ratio of switch S2 and turning offswitch S6 completely, controller 26 can cause ES1 4 to provide power toDC current link 16 while bypassing ES2 6. By controlling switch S6 toits off state, current flowing through converter 10 flows through switchS7 and through diode D8 on its way to switch S3 of converter 8.Alternatively, by controlling switch S7 to its off state, currentflowing through converter 10 flows through diode D5 and switch S6 on itsway to switch S3 of converter 8. Likewise, by controlling switch S6 toits on state and switch S2 to its off state, controller 26 can cause ES26 to provide current to DC current link 16 while bypassing ES1 4.

In addition, by controlling switches S2 and S6 (with S3 and S7 ON) orswitches S3 and S7 to their off states (with S2 and S6 ON), controller26 can cause both ES1 4 and ES2 6 to stop supplying current to DCcurrent link 16 when it is desired to halt operation in the motoringmode.

FIG. 3 illustrates a schematic diagram of the traction system shown inFIG. 1 operating in a regenerative braking mode in accordance with anembodiment of the invention. In this embodiment, controller 26 is showncontrolling the transfer of energy from load 20 during a regenerativebraking event to ES2 6. By maintaining all switches S1-S8 in their offstates, current flows from second bus 14 through diode D5, ES2 6, diodeD8, diode D1, ES1 4, and diode D4 to first bus 12. In this manner, ES1 4and ES2 6 may both be recharged during the regenerative braking event.

However, by controlling either switch S2 or switch S6 to their onstates, it is possible to respectively bypass ES1 4 or ES2 6 when it isdesired to avoid recharging either ES1 4 or ES2 6.

While converters 8 and 10 are each shown in an H-bridge configurationhaving four switches and four diodes, embodiments of the inventioncontemplate removing one or more of the switches or diodes for cost andweight reduction savings benefits should it be determined that theswitches or diodes to be removed will not have current flowingtherethrough in any of the control modes programmed into controller 26or in any other currents-flowing mode.

FIG. 4 illustrates another embodiment of modular stacked DC architecturetraction/propulsion system 2. As shown, in addition to the componentsillustrated in FIG. 1, traction system 2 of FIG. 4 includes anadditional bi-directional DC-to-DC converter 28 coupled between DCcurrent link 16 and load 20. Similar to converters 8 and 10, converter28 includes a plurality of power switches S9-S12 coupled in ananti-parallel arrangement with a plurality of diodes D9-D12. Controller26 is additionally coupled to switches S9-S12 of converter 28 to controlconversion of the current on DC current link 16 to a voltage that issupplied to load 20 and to control conversion of a voltage from load 20to a current suitable for supplying to DC current link 16 for chargingone or both of ES1 4 and ES2 6. In this embodiment, inverter 22 is avoltage source inverter configured to convert the voltage suppliedthereto to an energy suitable for driving motor 24.

FIG. 5 illustrates a schematic diagram of the traction system shown inFIG. 4 operating in a motoring mode in accordance with an embodiment ofthe invention. In this embodiment, controller 26 controls the transferof energy from one or both of ES1 4 and ES2 6 to load 20 for operatingmotor 24 in the motoring mode. As illustrated in bold, controller 26allows the transfer of energy from both ES1 4 and ES2 6 to DC currentlink 16 by maintaining switches S1, S4, S5, and S8 in an off state andby controlling the duty ratio of switches S2, S3, S6, and S7. In thismanner, current flowing from second bus 14 of DC current link 16 flowsalong a path through switch S7, ES2 6, switch S6, switch S3, ES1 4, andswitch S2 and to first bus 12 of DC current link 16. The voltagesupplied to load 20 is regulated by controlling the duty ratio of switchS12. When S12 is on, the current from first bus 12 flows through switchS12 and diode D11 to second bus 14. When switch S12 is off, the currentfrom first bus 12 flows through diode D10, load 20, and diode D11 tosecond bus 14.

While the embodiment illustrated in FIG. 5 shows control of switches S2,S3, S6, and S7 of converters 8 and 10 to control the transfer of energyfrom both ES1 4 and ES2 6, by controlling the duty ratio of switches S2and S6, controller 26 can also regulate the power to be drawn only fromone of the energy storage elements, i.e., either from ES1 4 or from ES26. For example, by controlling duty ratio of switch S2 and turning offswitch S6 completely, controller 26 can cause ES1 4 to provide power toDC current link 16 while bypassing ES2 6. By controlling switch S6 toits off state, current flowing through converter 10 flows through switchS7 and through diode D8 on its way to switch S3 of converter 8.Alternatively, by controlling switch S7 to its off state, currentflowing through converter 10 flows through diode D5 and switch S6 on itsway to switch S3 of converter 8. Likewise, by controlling switch S6 toits on state and switch S2 to its off state, controller 26 can cause ES26 to provide current to DC current link 16 while bypassing ES1 4.

In addition, by controlling switches S2 and S6 (with S3 and S7 ON) orswitches S3 and S7 to their off states (with S2 and S6 ON), controller26 can cause both ES1 4 and ES2 6 to stop supplying current to DCcurrent link 16 when it is desired to halt operation in the motoringmode.

FIG. 6 illustrates a schematic diagram of the traction system shown inFIG. 4 operating in a regenerative braking mode in accordance with anembodiment of the invention. In this embodiment, controller 26 is showncontrolling the transfer of energy from load 20 during a regenerativebraking event to ES2 6. During the regenerative braking event,controller 26 controls motor 24 to operate in a generator mode to slowdown or decelerate the vehicle, for example. In the generator mode,motor 24 generates energy that can be supplied to converter 28 throughinverter 22. Energy from load 20 can be caused to flow into ES2 6 bycontrolling the duty ratio of switch S9 (or S12) with S12 (or S9) turnedon completely. Accordingly, the current flowing from switch S9 flowsthrough second bus 14 of DC current link 16 and through diode D5 toreach ES2 6. From there, the current flows through diode D8 and on toconverter 8. By controlling switch S3 into its on state, theregenerative current bypasses diode D1 and ES1 4 on its path to firstbus 12 of DC current link 16 through diode D4.

In another embodiment, by controlling switch S3 into its off state,controller 26 can cause the regenerative breaking current to flowthrough diode D1 to charge ES1 4 in addition to charging ES2 6. Further,by controlling switch S7 into its on state, the regenerative current maybe forced to bypass ES2 6 on its way through diode D8 and on toconverter 8.

FIG. 7 illustrates a schematic diagram of the traction system shown inFIG. 4 operating in an energy transfer mode in accordance with anembodiment of the invention. In this embodiment, controller 26 is showncontrolling the transfer of energy from ES2 6 to ES1 4 while bypassingthe transfer of energy to load 20. As illustrated in bold, controller 26allows the transfer of energy from ES2 6 to ES1 4 by maintainingswitches S1-S5, S8, and S10-S12 in their off state; switches S9, S6 intheir on state; and by controlling duty ratio of switch S7. In thismanner, current flowing from ES2 6 flows along a path through switch S6and diode D1 to reach ES1 4. From ES1 4, the current returns to ES2 6via the path flowing through diode D4, first bus 12 of DC current link16, diode, D10, and switches S9 and S7.

In addition, by controlling switch S9 to its off state and bycontrolling inverter 22, controller 26 may cause both an energy transferfrom ES2 6 to ES1 4 in an energy transfer mode while simultaneouslycausing an energy transfer from ES2 6 to load 20 in a motoring mode. Inthis manner, if the state of charge of ES1 4 is reduced below a desiredthreshold level, ES2 6 may be used to begin or maintain motoring of thevehicle while simultaneously raising the state of charge of ES1 4 to adesired level.

In an embodiment where ES1 4 is configured to transfer its energy to ES26, the flow of current through ES1 4 and ES2 6 as depicted in FIG. 5 maybe reversed for such a transfer by controlling switches S6 and S7 totheir off states and switches S2 and S3 to their on states.

As illustrated in FIGS. 5-7, DC current link 16 is uni-directional. Thatis, the current in DC current link 16 flows from converter 8 toconverter 28 and from converter 28 to converter 10 in all of theembodiments described above. Because of this, active control of some ofthe switches S1-S12 to their on states does not occur. For example,switches S1, S4, S5, S8, S10, and S11 are not switched to their onstates to avoid the opposite flow of current in DC current link 16 inthe embodiments described herein. As such, switches S1, S4, S5, S8, S10,and S11 may be removed to reduce circuitry for cost and weight reductionsavings benefits if desired.

Therefore, according to one embodiment of the invention, a propulsionsystem includes an electric drive, a direct current (DC) linkelectrically coupled to the electric drive, and a first DC-DC convertercoupled to the DC link. A first energy storage device (ESD) iselectrically coupled to the first DC-DC converter, and a second DC-DCconverter is coupled to the DC link and to the first DC-DC converter.The system also includes a second energy storage device electricallycoupled to the second DC-DC converter and a controller coupled to thefirst and second DC-DC converters and configured to control a transferof energy between the first ESD and the DC link via the first and secondDC-DC converters.

According to another embodiment of the invention, a method of assemblinga control system includes coupling a first energy storage device (ESD)to a first DC-DC converter, coupling the first DC-DC converter to a DClink, and coupling a second ESD to a second DC-DC converter. The methodalso includes coupling the second DC-DC converter to the first DC-DCconverter and to the DC link, coupling the DC link to an electric drive,coupling a controller to the first and second DC-DC converters, andconfiguring the controller to cause the first and second DC-DCconverters to transfer energy between the first ESD and the DC link.

According to another embodiment of the invention, an energy storagearrangement for an electrically powered system includes a first energystorage device coupled to a first DC-DC converter, a second energystorage device coupled to a second DC-DC converter, and a DC link. TheDC link includes a first bus coupled to the first DC-DC converter and asecond bus coupled to the second DC-DC converter. The arrangementfurther includes a controller coupled to the first and second DC-DCconverters and configured to cause a current to flow from the second busto the first bus through the first energy storage device and through thefirst and second DC-DC converters.

This written description uses examples to disclose the invention,including the best mode, and also to enable any person skilled in theart to practice the invention, including making and using any devices orsystems and performing any incorporated methods. The patentable scope ofthe invention is defined by the claims, and may include other examplesthat occur to those skilled in the art. Such other examples are intendedto be within the scope of the claims if they have structural elementsthat do not differ from the literal language of the claims, or if theyinclude equivalent structural elements with insubstantial differencesfrom the literal languages of the claims.

What is claimed is:
 1. A propulsion system comprising: an electricdrive; a direct current (DC) link comprising a first bus and a secondbus coupled to the electric drive; a first DC-DC converter comprising afirst output bus coupled to the first bus of the DC link; a second DC-DCconverter comprising: a first output bus coupled to a second output busof the first DC-DC converter; and a second output bus coupled to thesecond bus of the DC link; a first energy storage device (ESD) coupledto the first DC-DC converter; and a second ESD coupled to the secondDC-DC converter.
 2. The propulsion system of claim 1 wherein a firstterminal of the first ESD is coupled to a first terminal of the firstDC-DC converter and a second terminal of the first ESD is coupled to asecond terminal of the first DC-DC converter; and wherein a firstterminal of the second ESD is coupled to a first terminal of the secondDC-DC converter and a second terminal of the second ESD is coupled to asecond terminal of the second DC-DC converter.
 3. The propulsion systemof claim 1 wherein the first DC-DC converter electrically connects thesecond ESD to the first bus of the DC link.
 4. The propulsion system ofclaim 3 wherein the second DC-DC converter electrically connects thefirst ESD to the second bus of the DC link.
 5. The propulsion system ofclaim 1 wherein the first DC-DC converter and the second DC-DC convertercomprise bi-directional DC-DC converters.
 6. The propulsion system ofclaim 1 further comprising a third DC-DC converter coupled between theelectric drive and the first and second DC-DC converters.
 7. Thepropulsion system of claim 1 further comprising a controllerelectrically coupled to the first DC-DC converter and the second DC-DCconverter to selectively control operation thereof.
 8. The propulsionsystem of claim 1 wherein the first DC-DC converter comprises a firstplurality of switches and a first plurality of diodes; and wherein thesecond DC-DC converter comprises a second plurality of switches and asecond plurality of diodes.
 9. The propulsion system of claim 8 whereinthe controller is programmed to transfer energy from the first ESD andthe second ESD to the electric drive by: controlling a first subset ofthe first plurality of switches to operate in an off state; controllinga second subset of the first plurality of switches to operate in an onstate; controlling a first subset of the second plurality of switches tooperate in an off state; and controlling a second subset of the secondplurality of switches to operate in an on state.
 10. The propulsionsystem of claim 8 wherein the controller is programmed to transferenergy from the electric drive to the first ESD and to the second ESDby: controlling the first plurality of switches to operate in an offstate; and controlling the second plurality of switches to operate in anoff state.
 11. The propulsion system of claim 8 wherein the controlleris programmed to transfer energy from the electric drive to only one ofthe first ESD and the second ESD by controlling only one switch of oneof the first plurality of switches and the second plurality of switchesto operate in an on state.
 12. The propulsion system of claim 11 whereinthe controller is programmed to transfer energy from the electric driveto the first ESD by controlling one of the first plurality of switchesto operate in an on state.
 13. The propulsion system of claim 11 whereinthe controller is programmed to transfer energy from the electric driveto the second ESD by controlling one of the second plurality of switchesto operate in an on state.
 14. The propulsion system of claim 8 whereinthe first DC-DC converter comprises four switches and four diodesarranged in an H-bridge configuration; and wherein the second DC-DCconverter comprises four switches and four diodes arranged in anH-bridge configuration.
 15. A method of manufacturing a propulsionsystem comprising: coupling an electric drive to a first bus and to asecond bus of a DC link; coupling a first DC-DC converter to theelectric drive by coupling a first output bus of the first DC-DCconverter to the first bus of the DC link; coupling a second DC-DCconverter to the first DC-DC converter by coupling a first output bus ofthe second DC-DC converter to a second output bus of the first DC-DCconverter; coupling the second DC-DC converter to the electric drive bycoupling a second output bus of the second DC-DC converter to the secondbus of the DC link; coupling a first energy storage device (ESD) to thefirst DC-DC converter; and coupling a second ESD to the second DC-DCconverter.
 16. The method of claim 15 further comprising electricallycoupling a controller to the first and second DC-DC converters, thecontroller programmed to selectively control switching thereof.
 17. Themethod of claim 15 further comprising coupling a third DC-DC converterbetween the electric drive and the first and second DC-DC converters.18. The method of claim 15 wherein coupling the first ESD to the firstDC-DC converter comprises: coupling a first terminal of the first ESD toa first terminal of the first DC-DC converter; and coupling a secondterminal of the first ESD to a second terminal of the first DC-DCconverter; and wherein coupling the second ESD to the second DC-DCconverter comprises: coupling a first terminal of the second ESD to afirst terminal of the second DC-DC converter; and coupling a secondterminal of the first ESD to a second terminal of the second DC-DCconverter.
 19. An energy storage arrangement for a vehicle propulsionsystem comprising: a direct current (DC) link coupleable to an electricdrive; a pair of bi-directional DC-DC converters each having a firstterminal, a second terminal, a third terminal, and a fourth terminal;and a pair of energy storage devices, wherein one energy storage deviceis coupled between the first and second terminals of one bi-directionalDC-DC converter and the other energy storage device is coupled betweenthe first and second terminals of the other bi-directional DC-DCconverter; wherein the third terminal of each bi-directional DC-DCconverter is coupled to the DC link; and wherein the fourth terminal ofone bi-directional DC-DC converter is directly coupled to the fourthterminal of the other bi-directional DC-DC converter.
 20. The energystorage arrangement of claim 19 wherein the third terminal of onebi-directional DC-DC converter is directly coupled to a first bus of theDC link; and wherein the third terminal of the other bi-directionalDC-DC converter is directly coupled to a second bus of the DC link.